In recent years, it is apprehended that minute particles (nanoparticles) having 10 nm to several 100 nm in diameter which are being used in such various industries as cosmetics might cause damage on human health with the infiltration into human bodies or their cells, as the result of which the USA and European countries have started regulating the use of such particles for commercial use. The use of such particles is regulated based on their diameter in France and other countries while it is regulated based on their toxicity in the USA in addition to their diameter. Accordingly, it is necessary to measure the three-dimensional particulate shape and evaluate the particulate species (particle materials).
As to measuring the three-dimensional particulate shape of such particles, it is to be standardized by means of such microscopes employing a charged particle beam (hereinafter, referred to as charged particle microscopes) as scanning probe microscopes (SPM) and scanning electron microscopes (SEM). According to the conventional steps of measuring such particles, to begin with, the sample powders extracted from the powdered raw nanoparticle material is scaled; and then such sample powders are dispersed in a solution from which impurities are removed so as to be changed into a suspended solution. In this regard, the particulate species included in nanoparticles, the average particle size, the standard deviation of particle sizes and the particle size distribution for each particulate species are all unknown, and there are many particles whose three-dimensional shape is varied or other than complete sphere. As one example, the measuring steps according to an atomic force microscope (AFM) which is one of the SPMs are as follows, which steps are also applied to the charged particle microscopes.
(1) Step of preparing a suspended solution of nanoparticles to be measured
(2) Step of dropping such suspended solution onto a flat substrate
(3) Step of drying a droplet
(4) Step of subjecting a probe profile to AFM measurement employing a standard sample
(5) Step of selecting a location of the dried droplet which is optimal with respect to the density of nanoparticles through AFM measurement
(6) Step of measuring three-dimensional shape images of nanoparticles whose number is sufficient for carrying out statistical processing
(7) Step of smoothing three-dimensional shape images to remove noises from the shape images
(8) Step of particulate analysis: calculating an average particle size of nanoparticles to be measured, the standard deviation of particle sizes and the particle size distribution
The spatial resolution in the order of 1 nm with respect to the asperity measurement (height measurement) is feasible with SPMs. However, with the three-dimensional measurement through SPMs, the particulate size with respect to the transversal direction (i.e. direction in parallel with the substrate surface on which nanoparticles are fixed) is measured with enlargement according to the shape of the tip end portion of the probe, which is generally called as probe shape effect. Accordingly, it requires that the measured information be corrected by some measures. With the charged particle microscopes, such method of obtaining information on the three-dimensional particulate shape is employed as e.g. tilting the substrate on which nanoparticles are fixed with respect to the direction in which the charged particle beam is made incident. However, such SPMs and charged particle microscopes have it in common that the resulting images do not always correctly represent the shape of a sample due to e.g. the intensity profile of the charged particle beam and the secondary charged particle generation mechanism.
In Patent Literature 1, there is disclosure on a measured shape correction means to detect the state of the probe based on the measured result of a standard sample whose shape is known and to rectify the measured result of a sample surface based on the detected state of the probe, in addition to which information on the three-dimensional shape of a sample are obtained by alternately measuring a standard sample and a sample to be measured so as to rectify such probe shape effect.
In Patent Literature 2, there is disclosure on a method for measuring the surface roughness of nanoparticles including the steps of dropping a solution in which silica nanoparticles are dispersed onto the mica substrate having the amino group on the surface; drying the droplet to fix the same on the substrate; measuring the three-dimensional shape image of the silica nanoparticles by means of an atomic force microscope (AFM); and calculating the arithmetic mean roughness, thereby, allowing the surface shapes of the nanoparticles to be distinguished from one another with numeric values.